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A button or lever on the camera or exposure meter used to lock the automatically measured light reading into a memory while you recompose.This is ideal for backlit subjects because you can move closer to the subject, take a reading without any background in the view, lock the exposure and move back to the original position to take the photo.It's also fine for landscapes when there's an expanse of bright sky. Point the camera down to the ground, lock the exposure, recompose and shoot.Use this to bias the exposure to one area of the scene or to maintain a consistent reading when lighting conditions are variable.